Field of the Invention
The present invention relates to a method of producing a hollow foamed plastic body made of polyethylene or polypropylene in an extrusion, blow-molding process with a single-screw extruder and with a blow-molding tool, in which the plastic material is foamed by means of a chemical blowing agent and a pre-molded tube blank formed from the foamed plastic material is blow-molded in the blow-molding tool.
In the course of attempts to reduce material outlay for hollow plastic bodies, in particular in packaging, it is known to produce packaging containers (bottles) from foamed plastic material. Packaging containers of this type are frequently used in areas of application in which a smooth exterior surface is an essential functional feature, so as to present an attractive appearance of the product. However, the foamed hollow bodies produced in extrusion blow-molding have an irregular bulging of the exterior surface caused by gas bubbles in the foam. The distribution of the gas bubbles in the wall of the foamed hollow bodies is also frequently not sufficiently homogeneous, which results in irregular gas cavities and which affects in turn the exterior surface as well as the mechanical properties of the wall. These effects are particularly serious where the wall thicknesses of the hollow bodies are in the range of less than 1.5 mm. In order to avoid this drawback in the case of extrusion blow-molded foamed hollow bodies, a compact, smooth outer skin for example is extruded on in the coextrusion process. Other methods, such as for example in accordance with DE 1504359-A, suggest producing a compact outer skin by rapid cooling of the extruded tube on the outside thereof before the blow-molding process. In this case, an attempt is made at the same time to regulate an uncontrolled foam expansion on the tube.
In a further publication PCT/EP96/05485 it is proposed to produce a smooth outer skin by encasing a container with a smooth container decoration.
All the steps and methods described have the drawback that special applications have to be performed with respect to the smooth outer skin, which increase costs in such a way that a substantial proportion of the cost advantage is offset by the foamed hollow body.
In a further publication DE 19525198-A a method is described which is intended to allow an extrusion blow-molded foamed hollow body to be produced whilst maintaining specific parameters. For this purpose, a foaming agent is admixed with a commercially available PE granulate. This usually takes place as a batch granulate on the basis of PE with a grain size of from 3 to 5 mm or as a powder which is admixed with the PE granulate. In addition, the plastics-material melt is heated to from 20 to 30.degree. above the melting temperature of the compact plastics material and a pressure of the blowing air of from 0.5 to 2.5 bar is applied. In this case, the blow-molding tool is cooled to a temperature of from 7 to 15.degree. C. When setting these parameters, which only represent the prior art, although a foamed hollow body can be produced, it does not meet the requirements with respect to the desired properties, in particular the smooth outer surface and the fine distribution of the gas bubbles in the plastics-material matrix.